In an internal combustion engine provided with a NOx absorbing reduction catalyst (hereinafter, referred to as absorbing catalyst) which locates in an exhaust gas passage, when NOx is absorbed by the absorbing catalyst up to a certain level, the NOx thus absorbed is reduced and removed by executing an operation called a rich spike by which an air-fuel ratio is switched temporarily and rapidly from leanness to richness.
As an invention of method for recovering the absorbing catalyst by the rich spike, there is a Japanese Laid-Open Patent Publication No. 2000-45752 (Method for Cleaning NOx Absorbing Reduction Catalyst in Internal Combustion Engine), the applicant of which is the same of the present application. In the invention of the Japanese Laid-Open Patent Publication No. 2000-45752, a consideration is given to a point that an absorbing ability of the absorbing catalyst is fully realized by taking the following two steps. As a first step, a possible absorbing capacity of NOx by the absorbing catalyst is monitored. As a second step, when (or before) the accumulated value of the NOx flowing in the absorbing catalyst reaches a possible absorbing capacity, the operation of the rich spike is executed to clean the absorbing catalyst.
The absorbing catalyst is, however, poisoned by a sulfuric component included in the exhaust gas, and the absorbing catalyst deteriorates with time passing so that the possible absorbing capacity of NOx decreases. As a method for removing the sulfuric component from the poisoned absorbing catalyst, there is a Japanese Laid-Open Patent Publication No. 2000-8909 (Method for Controlling Internal Combustion Engine), the applicant of which is the same of the present application. In the invention of the Japanese Laid-Open Patent Publication No. 2000-8909, when a predetermined time (100 minutes, for example) elapses while the internal combustion engine is operating, the air-fuel ratio is switched over from the leanness to the richness, and the absorbing catalyst is recovered by operating it during a predetermined time (approximately 100 minutes) under a condition that the temperature of the exhaust gas is more than 600° C.
In this way, according to the conventional art, to which degree the absorbing catalyst is poisoned by the sulfuric component is estimated only from the operation time of the internal combustion engine. That is, the recovering work is performed mechanically when the predetermined time passes, without monitoring the actual deteriorating condition of the absorbing catalyst. The “deterioration of the absorbing catalyst” means decrease of performance of cleanup of the absorbing catalyst on the basis of the poisoning by the sulfuric component and by heat.